The present invention relates generally to communications receivers, and more particularly to a proactive gain control system for communications receivers.
A gain control system is typically used to control an input signal level of a communication receiver. Environmental changes can change the input signal level. For example, in a cellular communications system, the input signal level may change by 20 dB within a few milliseconds due to a movement of mobile units or a channel fading. A change of a traffic profile can also drastically change the input signal level. This traffic profile includes the data traffic attributes, such as a modulation type of the input signal, a number of traffic channels for the input signal, a coding scheme of the input signal and a data transfer rate of the input signal. The change of the signal level caused by the environmental changes is relatively gradual as opposed to those caused by the traffic profile changes.
A communications receiver has a limited dynamic range for receiving input signals. A change of the input signal level caused by the environmental changes or the traffic profile changes can go beyond the limited dynamic range of the communications receiver. Thus, most of the communication receivers have some sort of gain control systems. One purpose of the gain control system is to reduce the receiver gain when the input signal level is too high, and to increase the receiver gain when the input signal level is too low. Thus, the input signal can be maintained at an optimal level within the dynamic range of the receiver.
There are many conventional automatic gain control (AGC) circuits in use today. Most of these AGC circuits are the reactive type of gain controllers. They typically include a signal strength detector to detect the currently received signal level, a comparator that compares the received signal level to a threshold level, and a control circuit that adjusts the receiver gain to reduce a signal error generated by the comparator. These reactive AGC circuits use a kind of feel-first-and-then-react approach. However, the reactive AGC circuits do not cope well with a drastic change of the input signal caused by the traffic profile changes.
Therefore, desirable in the art of gain control systems for communications receivers are circuit designs that provide a proactive receiver gain control scheme taking into account a drastic change of the input signal caused by traffic profile changes.